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Construction of externallyinsulated wall.
Courtesy of Bovis Lend Lease. |
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Walls using these systems have become popular in recent years and are normally constructed on site in a number of packages, which will generally include three stages.
- Construction of a back wall, commonly using steel studs sheathed externally with fibre-cement boards and internally with a vapour barrier and dry lining. The wall construction may itself be carried out by several separate sub-contractors.
- Window installation.
- Application of the external insulation and a render finish. The render finish may be a cement-based material with a metal carrier or a thin-fibre-reinforced, polymer-based material.
The overall performance of the wall will depend on the ability of the various layers to work together and in particular the design and construction of interfaces. Many of the external insulation systems on the market have independent, third-party assessment in the form of Agrément certificates from the BBA. However, these certificates are for use of the systems on masonry or concrete back walls rather than steel stud walls.
Problems with this form of construction in other parts of the world, notably the USA, Canada and New Zealand, include mould growth, corrosion of metal, and rot of timber components due to rain penetration and condensation. There is particular concern when these systems are used for apartment buildings, where internal humidity levels may be high, increasing the risk of condensation. A long building life is required to maintain the value of the apartment owner’s investment and major external maintenance may be difficult due to split ownership of the building.
The Technical Notes are
- TN 59 Introduction to externally rendered cladding systems;
- TN 60 Performance of externally rendered cladding systems.
They highlight the need for an engineered approach to the use of these systems with a single body taking responsibility for the design and the construction of the complete wall, including interfaces with adjacent construction. The use of a secondary defence system to resist rain penetration is also highlighted.
For further information please contact Brenda Apted at CWCT (01225 386506; E-mail cwct@bath.co.uk).
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